This invention relates to improvements in or relating to a grab bucket for grab dredger. This grab dredger, popularly called Priestman, has one or more cranes mounted on the dredger to suspend one or more grab buckets. The grab bucket which is freely opened or closed is submerged into water to dig bottom ground and then lifted with earth contained therein. The grab dredger is generally used for dredging mud and sand, and it is also used with a crusher or blasting method for dredging soft and hard rocks. However, it is well known that operating efficiency in dredging hard soil and gravels is less than that in dredging the above-mentioned kind of soil. To overcome this problem, the bucket weight is greatly increased -- generally five times the normal weight, which results in the significantly increased construction cost.